Respiration in Invertebrate Animals. 31 



moved in structure far beyond the sphere of naked vision. Since 

 however the individual parts of a pectinibranchiate gill con- 

 stitute under all circumstances sheets whose opposite faces are 

 more or less smooth, or more or less corrugated and folded, a 

 little manipulative skill will be required to enable the student to 

 put to the test of personal observation the particulars comprised 

 in the following description. 



The language commonly used by malacologists in describing 

 the gills of this order of MoUusks is calculated to lead to many 

 very false conceptions. They are first said to be "plumes.'^ A 

 ' plume ' or feather is 6i-pectinate, that is, it consists of a stem 

 bearing ' barbs ' on either side. Such a word, therefore, conveys 

 to the mind an untrue image of the real object. The word 

 " pectinate ^^ is nearer, but still very erroneous, and very inade- 

 quate as an illustrative analogue. A leaf of the gill of a pec- 

 tinibranchiate mollusk resembles in figure much more nearly a 

 ' fan ' than the tooth of a comb. A gill would be a series of 

 fans laid side by side. It should accordingly be defined rather 

 as flabelliform (fig. 4) than pecten-iform. The laminae of this 

 gill are comparable to a *fan' moreover in this remarkable 

 particular — they are capable of being closed and opened under 

 the action of muscles. In fact, in other respects they constitute 

 an apparatus immeasurably more beautiful and complex than 

 it has hitherto entered into the dreams of naturalists to con- 

 ceive. 



The branchia (fig. I «, a) of every genus of this order is seated 

 on a fixed base which forms a part of the roof of the respiratory 

 chamber [e, e). In this respect it diff*ers from the breathing 

 organ of the Tectinibranchs, and resembles that of the Cyclo- 

 branchs. But the pectinibranchiate gill is distinguished from 

 that of every branchiferous Gasteropod, and is brought near to 

 that of a Lamellibranch by a curious incident of structural 

 mechanism. Each and every leaf of the gill is stifiened and 

 strengthened at one of its free borders by the insertion into its 

 substance of a whalebone-like process of rigid cartilage (fig. 4 a, a, 

 & fig. 7). The presence of this process imparts to this edge of 

 the organule a thick straight appearance which stands in obvious 

 opposition to the floating and flexible character of the other 

 margin (fig. 4 6). As this process of cartilage is concealed in 

 the substance of the dorsal border of the leaf, and embraced by 

 a dense ciliated membrane, it can only be detected by tearing 

 up the whole leaf into pieces by means of needles. Viewed on 

 its flat surface the lamina presents a triangular outline {a, b, c, d). 

 This is more or less the figure of the branchial laminae through- 

 out the entire Pectinibranchiate group. The terminology ap- 

 plicable in one genus will serve to designate the homologous 



